Pore-scale insight into the impact of heterogeneity in porous media on CO2–oil immiscible displacement

CCUS (carbon dioxide capture, utilization, and storage) technology constitutes a pivotal solution for achieving net-zero carbon emissions. As an important implementation pathway of CCUS, CO2–Enhanced Oil Recovery (EOR) can not only enable CO2 storage but also substantially improve the hydrocarbons production. The pore structure of reservoir porous media greatly impacts microscopic CO2 displacement, which is crucial for CO2–EOR. Pore-structure heterogeneity, a dominant factor in CO2 flooding, exerts pivotal control over displacement dynamics, which remains unclear. To bridge gaps, this study employs pore-scale numerical simulations coupling the Navier–Stokes (N-S) equations with phase-field method to elucidate heterogeneity effects on immiscible CO2–oil displacement. The recovery in porous media, the types of residual oil, and the morphology of the CO2–oil displacement front are systematically investigated across varying capillary numbers (Ca) and wettability conditions. The results demonstrate that enhanced heterogeneity induces preferential flow channel development in porous media, which accelerates CO2 breakthrough time and impairs oil recovery efficiency. In highly heterogeneous models, Ca effects are diminished with displacement front shape and residual oil distribution dominated by heterogeneity, and oil recovery even decreases anomalously with increasing Ca. Contact angle (θ) impact on heterogeneity depends on wettability: under oil-wet conditions (θ < π/2), higher θ mitigates CO2 channeling from heterogeneity, while under CO2-wet conditions (θ > π/2), higher θ exacerbates such channeling. This study reveals the critical role of pore-scale heterogeneity in CO2–oil displacement, which is significant for optimizing CO2–EOR efficiency.